This invention relates to in-circuit test fixtures in general and more specifically to a high density in-circuit test fixture for fine pitch, high density circuits.
In-circuit test (ICT) probes are known in the art and are typically used to test printed circuit assemblies (i.e., PCAs) to ensure the proper functioning and operation thereof. If the printed circuit assembly or PCA successfully completes the test, it may be passed on for incorporation into the appropriate sub-assembly or into the final product, as the case may be. If, on the other hand, the PCA fails the test, it may either be repaired or scrapped.
One type of ICT probe that is commonly used today comprises a two dimensional grid or array of elongated, spring-loaded test pins or xe2x80x9cpogo pins.xe2x80x9d The test pins or pogo pins may be arranged on a supporting board or substrate so that the pins extend generally outward from the surface of the board. The arrangement of the test pins on the board is similar to a so-called xe2x80x9cbed of nailsxe2x80x9d and such ICT probes are often referred to as xe2x80x9cbed of nailsxe2x80x9d probes or testers. When placed against the PCA to be tested, the various pogo pins comprising the ICT probe make contact with various circuit nodes on the PCA. Test equipment and/or circuitry electrically connected to the various pogo pins in the ICT probe thereafter tests the operation and function of the various components on the PCA to ensure proper operation.
While such two dimensional pin grid arrays or xe2x80x9cbed of nailsxe2x80x9d testers are known and have been used with success in the past, the increasing area density (i.e., circuit node pitch) on newer PCAs makes it difficult to test all the desired circuit nodes contained on the PCA. For example, while most currently available bed of nails testers or probes have pins that are spaced on 50 mil (0.05 inch) centers, most of the new PCAs contain at least portions where the circuit node pitch is less than 50 mil. While it is possible to reduce the pin pitch somewhat, the physical dimensions of even the smallest currently available pogo pins precludes a pin pitch of less than about 35 mils. Since many of the newer PCAs may contain areas where the circuit node pitch is on the order of 10 mils to 1 mil, or even less, the currently available ICT bed of nails testers are simply not able to test all of the circuit nodes contained on the higher density portions of the PCA. In fact, with current ICT and PCA designs, it is typical that only about 70% of the circuit nodes contained on the PCA can be tested. Such limited testing means that a given PCA may pass the testing stage even though in fact it may be defective.
Another problem with currently available bed of nails ICT test fixtures is that they are difficult and time consuming to construct. The individual pogo pins of the testers are usually manually wired together and to the various required input/output nodes or terminals that are used to connect the ICT probe to the external test devices and circuitry. Therefore, even if the pitch of the pins could be reduced to 35 mils or less, such a high density pin arrangement would make it extremely difficult, if not impossible, to manually wire together the various pins according to currently used methods.
Consequently, a need exists for an ICT probe that is capable of accessing substantially all of the circuit nodes contained on the newer, high density PCAs currently being fabricated. Ideally, such an improved ICT probe should be capable of testing PCAs having circuit node pitches of 10 mils or even 1 mil. Additional advantages could be achieved if such a high density ICT probe substantially reduced or eliminated the need to hand wire together the various probe points, thereby substantially shortening the time required to fabricate an ICT probe for a given PCA design.
A test fixture for performing in-circuit testing of a printed circuit assembly may comprise a board having a front surface and a back surface. A probe assembly also having a front surface and a back surface is mounted to the board so that the back surface of the probe assembly is adjacent the front surface of the board. The probe assembly includes at least one front surface contact pad positioned on the front surface of the probe assembly that is electrically connected to at least one back surface contact pad positioned on the back surface of the probe assembly. A first board pad positioned on the front surface of the board makes electrical contact with the back surface contact pad on the back surface of the probe assembly. An electrical conductor operatively associated with the board electrically connects the board pad to an input/output pad that is also provided on the board.
Also disclosed is a method for testing a printed circuit assembly that comprises the steps of: Providing a board with a probe assembly thereon, the probe assembly having at least one electrically conductive element therein for conducting electricity from a front surface contact pad located on the probe assembly to an electrical conductor provided on the board, and placing the front surface of the probe assembly adjacent the printed circuit assembly so that the front surface contact pad makes electrical contact with a circuit node contained on the printed circuit assembly.